Method of forming microsphere having structural color

ABSTRACT

Provided is a method of forming a microsphere having a structural color, which includes providing a composition for generating a structural color including a curable material and magnetic nanoparticles dispersed in the curable material, forming an emulsion by adding the composition for generating a structural color to an immiscible solvent, arranging the magnetic nanoparticles located in the emulsion droplet of the curable material in a one-dimensional chain structure by applying a magnetic field to the emulsion, and fixing the chain structure by curing the emulsion droplet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 13/271,668filed Oct. 12, 2011, which is a continuation-in-part and claims thebenefit of International Application No. PCT/KR2010/002302 filed Apr.14, 2010, which in turn claims the benefit of U.S. ProvisionalApplication No. 61/169,260, filed Apr. 14, 2009, the disclosures ofwhich are incorporated by reference into the present application.

TECHNICAL FIELD

The described technology relates generally to a method of forming amicrosphere having a structural color.

BACKGROUND

Photonic crystal materials with a band gap property responsive toexternal stimuli have important applications in bio- and chemicalsensors, color paints and inks, reflective display units, opticalfilters and switches, and many other active optical components.Colloidal crystals, which can be produced conveniently byself-assembling uniform colloidal particles, have been particularlyuseful for making responsive photonic materials because activecomponents can be incorporated into the crystalline lattice during orafter the assembly process. The majority of research in the fieldtherefore has been focused on tuning the photonic properties ofcolloidal systems through changes in the refractive indices, latticeconstants, or spatial symmetry of the colloidal arrays upon theapplication of external stimuli such as chemical change, temperaturevariation, mechanical forces, electrical or magnetic fields, or light.However, wide use of these systems in practical applications is usuallyhampered by slow and complicated fabrication processes, limitedtunability, slow response to the external stimuli, and difficulty ofdevice integration.

Because the photonic band gap is highly dependent on the angle betweenthe incident light and lattice planes, an alternative route to tunablephotonic materials is to use external stimuli to change the orientationof a photonic crystal. For easy fabrication, actuation, and broaderapplications, it is highly desirable that the photonic crystals can bedivided into many smaller parts whose orientation can be controlledindividually or collectively as needed by using external stimuli.Photonic crystal microspheres, or “opal balls”, have been previouslydemonstrated by Velev et al. in a number of pioneering works by usingmonodisperse silica or polystyrene beads as the building blocks (Velev,O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240-2243;Rastogi, Melle, S.; Calderon, O. G.; Garcia, A, A.; Marquez, M.; Velev,O. D. Adv. Mater. 2008, 20, 4263-4268). The brilliant colors associatedwith these three dimensional periodic structures, however, cannot betuned due to lack of control over the orientation of the microspheres.Xia et al. have introduced magnetic components into a photonicmicrocrystal so that its diffraction can be changed by rotating thesample using external magnetic fields (Gates, B.; Xia, Y Adv. Mater,2001, 13, 1605-1608). However, it has not been demonstrated that one cansynthesize multiple copies of such microphotonic crystals, align themsynchronically, and collectively output uniform color signals.

SUMMARY

In one embodiment, a method of forming a microsphere having a structuralcolor is provided. The method includes: providing a composition forgenerating a structural color including a curable material and magneticnanoparticles in the curable material; adding the composition forgenerating a structural color to an immiscible solvent to form anemulsion; applying a magnetic field to the emulsion to align themagnetic nanoparticles located in an expulsion droplet of the curablematerial in one-dimensional chain structures; and curing the emulsiondroplet to fix the chain structures and form the microsphere.

In another embodiment, a microsphere having a structural color isprovided. The microsphere includes a solid matrix; and the magnetic;nanoparticles aligned in one-dimensional chain structures to exhibit astructural color within the solid matrix.

In yet another embodiment, a method of forming a microsphere having astructural color is provided. The method includes: A method of forming amicrosphere having a structural color comprising: applying a magneticfield to an emulsion system including superparamagnetic nanoparticles toform an ordered structure of the superparamagnetic nanoparticles; andapplying the magnetic field and simultaneously irradiating UV rays tothe emulsion to fix the ordered structure and form the microsphere.Here, the emulsion system includes droplets of a photocurable resincontaining the superparamagnetic nanoparticles and an immiscible solventin which the droplets are dispersed.

In still another embodiment, A display device comprising a microsphererotated by an external magnetic field is provided. Here, the microspherecontains an ordered structure of a photonic crystal due o the alignmentof the magnetic nanoparticles, and a diffraction angle of light passingthrough the microsphere is changed with rotation of the microsphere.

The Summary is pr vided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. The Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent application file contains at least one drawing executed incolor. Copies of this patent application publication with color drawingswill be provided by the Office upon request and payment of the necessaryfee.

The above and other features and advantages of the present disclosurewill become more apparent to those of ordinary skill in the art bydescribing in detail example embodiments thereof with reference to theattached drawings in which:

FIG. 1 is a diagram of a composition for generating a structural colorin accordance with an exemplary embodiment;

FIG. 2 is a diagram for explaining a principle of generating astructural color;

FIG. 3 is a diagram of a process of fixing a photonic crystal structureby curing a composition for generating a structural color;

FIG. 4 is a schematic of a synthetic procedure for the magnetochromaticmicrospheres;

FIG. 5 is a scanning electron microscopiy (SEM) image of Fe₃O₄ particlescoated with SiO₂ bedded in a PEGDA medium;

FIG. 6 shows schematic illustrations and optical microscopy images forthe magnetochromatic effect caused by rotating the chain-like photonicstructures in magnetic fields;

FIGS. 7A-7F illustrate optical microscopy images (500×) ofmagnetochromatic microspheres with diffractions switched between “on”(FIGS. 7A, 7C, 7E) and “off” (FIGS. 7B, 7D, 7F) states by using externalmagnetic fields, and wherein these microspheres were prepared using(FIGS. 7A, 7B) 127, (FIGS. 7C, 7D) 154, and (FIGS. 7E, 7F) 197 nmFe₃O₄@SiO₂ colloids;

FIG. 8 is a schematic illustration of the experimental setup forstudying the angular dependence of the diffraction property of themagnetochromatic microspheres;

FIG. 9 is a reflection spectrum and corresponding digital photo recordedfrom a single Fe₃O₄@SiO₂/PEGDA microsphere at different tilting angles;

FIG. 10 illustrates a series of microspheres, wherein FIG. 10(a)illustrates dark-field optical microscopy images of a series ofFe₃O₄@SiO₂/PEGDA microspheres with diameters from approximately 150 to 4μm, and wherein the larger microspheres were fabricated in mineral oiland smaller ones in silicone oil;

FIGS. 10(b)-10(d) are top view to side view SEM images of themicrospheres, showing some of the Fe₃O₄@SiO₂ particle chains aligned onthe surface along the longitudinal direction, and wherein it should benoted that a plurality of particle chains are embedded inside themicrospheres, with only ends occasionally observable in the top viewimage (b);

FIGS. 11A and 11B illustrate statistical diagrams showing the turningthreshold of field strength for Fe₃O₄@SiO₂/PEGDA microspheres withdifferent loadings of magnetic particles, wherein FIG. 11A is 8 and FIG.11B is 6 mg Fe₃O₄/ml PEGDA, and wherein the diagrams show the percentageof viewable area which is turned on at certain field strengths, and thecorresponding accumulative curves;

FIG. 12 illustrates schematic diagrams of the optical response ofFe₃O₄@SiO₂/PEGDA microspheres in a (FIGS. 12A, 12B) 1.22 and (FIGS. 12C,12D) 3.33 Hz vertical/horizontal alternating magnetic field, whereinHs/H0 is the ratio of reflection with H field to that without H field;and

FIGS. 13A-13I illustrate digital photos and reflection spectra of threetypes of Fe₃O₄@SiO₂/PEGDA microspheres loaded in 1.8 cm×1.8 cm×0.1 cmglass cells filled with PEG (M w=1500).

DETAILED DESCRIPTION

It will be readily understood that the components of the presentdisclosure, as generally described and illustrated in the Figuresherein, could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of apparatus and methods in accordance with the presentdisclosure, as represented in the Figures, is not intended to limit thescope of the disclosure, as claimed, but is merely representative ofcertain examples of embodiments in accordance with the disclosure. Thepresently described embodiments will be best understood by reference tothe drawings, wherein like parts are designated by like numeralsthroughout. Moreover, the drawings are not necessarily to scale, and thesize and relative sizes of the layers and regions may have beenexaggerated for clarity.

As used in the description herein and throughout the claims, thefollowing terms take the meanings explicitly associated herein, unlessthe context clearly dictates otherwise: the meaning of “a”, “an”, and“the” includes plural reference, the meaning of “in” includes “in” and“on”. It will also be understood that when an element or layer isreferred to as being “on” another element or layer, the element or layermay be directly on the other element or layer or intervening elements orlayers may be present. As used herein, the term “and/or” may include anyand all combinations of one or more of the associated listed items.

In accordance with an exemplary embodiment, a method of forming amicrosphere having a structural color is provided. First, a compositionfor generating a structural color including a curable material andmagnetic nanoparticles dispersed the curable material is provided.

Subsequently, the composition for generating a structural color is addedto an immiscible solvent to form an emulsion. A spherical particle maybe prepared using the composition for generating a structural color.When the composition for generating a structural color is mechanicallystirred in an immiscible solvent which is not easily mixed with thecompositions phase-separated, the composition for generating astructural color forms a spherical droplet and may be emulsified. As anexample, the immiscible solvent may be a non-polar solvent with highviscosity. For emulsification, a surfactant and/or cosurfactant may beadded.

Subsequently, the magnetic nanoparticles located in the emulsion dropletof the curable material are aligned in one-dimensional chain structuresby applying a magnetic field to the emulsion. When the magnetic field isapplied to a stirred solution of the emulsified composition forgenerating a structural color, the magnetic nanoparticles in theemulsified composition for generating a structural color are aligned inone-dimensional chain structures. With the magnetic field strength,inter-particle spacing between the magnetic nanoparticles may bechanged, and therefore a wavelength of diffracted light may be tuned.

Subsequently, a microsphere having a structural color may be formed bycuring the emulsion droplet to fix the chain structures. The curing maybe performed by exposing the curable material to UV rays. The structuralcolor may be continuously maintained through the fixation. The curablematerial may include a monomer or oligomer containing a crosslinkablesite. The microsphere contains an ordered structure of a photoniccrystal due to the alignment of the magnetic nanoparticles, and adiffraction angle of light passing through the microsphere may bechanged with rotation of the microsphere. The chain structures maygenerate a first color by forming the ordered structure of the photoniccrystal. In addition, the method of forming the microsphere may furtherinclude a process of changing the first color into a second color bychanging the diffraction angle of light passing through the microsphereby rotating the microsphere.

The method of forming the microsphere may further include a process ofdispersing the microspheres in a phase-changeable matrix. Thephase-changeable matrix may have a liquid phase and a solid phase. Ifthe phase-changeable matrix is in a liquid phase, an orientation of themicrosphere may be changed by adjusting an angle of the externalmagnetic field. As the phase-changeable matrix is changed into a solidphase by the solidifying procedure, the orientation of the microspheresmay be fixed. For example, after the microspheres prepared by the abovemethod are dispersed in a thermoplastic resin maintained in a liquidphase at a high temperature, a magnetic field is applied to exhibit astructural color, and the thermoplastic resin is solidified by loweringthe temperature, then the orientation of the microspheres may be fixedin a solid matrix. Examples of the phase-changeable matrix may include amaterial which can be reversibly switched between a liquid phase and asolid phase, such as paraffin, long-chain alkane, primary alcohol,polyethylene, polyethyleneglycol, polyethylene-block-polyethyleneglycolcopolymer and polyester.

In accordance with an exemplary embodiment magnetochromatic microspherescan be fabricated through instant assembly of superparamagnetic photoniccrystals inside emulsion droplets of UV curable resin followed by animmediate UV curing process to polymerize the droplets and fix theordered structures. When dispersed in the liquid droplets,superparamagnetic Fe₃O₄@SiO₂ core-shell particles self-organize underthe balanced interaction of repulsive and attractive forces to formone-dimensional chains, each of which contains periodically arrangedparticles diffracting visible light and displaying field-tunable colors.UV initiated polymerization of the oligomers of the resin fixes theperiodic structures inside the droplet microspheres and retains thediffraction property. Because the superparamagnetic chains tend to alignthemselves along the field direction, it is very convenient to controlthe orientation of such photonic microspheres and accordingly, theirdiffractive colors, by changing the orientation of the crystal latticerelative to the incident light using magnetic fields.

It can be appreciated that among potential external stimuli, a magneticfield has the benefits of contactless control, instant action, and easyintegration into electronic devices, though it has only been usedlimitedly in assembling and tuning colloidal crystals due to thecomplication of the forces that are involved. In accordance with anexemplary embodiment, a series of magnetically tunable photonic crystalsystems have been developed through the assembly of uniformsuperparamagnetic particles in liquid media with an exemplaryembodiment, the assembly of such photonic crystals includes theestablishment of balance between the magnetically induced dipolarattraction and the repulsions resulted from surface charge or otherstructural factors such as the overlap of solvation layers. This finelytuned dynamic equilibrium leads to the self-assembly of the magneticcolloids in the form of chain structures with defined internalperiodicity along the direction of external field, and also renders thesystem fast, fully reversible optical response across thevisible-near-infrared range when the external magnetic field ismanipulated.

The method of forming the microsphere may include a procedure ofchanging a structural color of the microsphere by rotating themicrosphere using an external magnetic field. It can be appreciated thatunlike “opal balls” whose orientation cannot be controlled, fixing ofphotonic crystals chains makes the microspheres magnetically “polarized”so that their orientation becomes fully tunable as chains of thesuperparamagnetic nanoparticles always tend to align along the externalmagnetic field direction. In addition, it appreciated that multiplecopies of photonic crystal microspheres can be fabricated in singleprocess, and their orientation can be synchronically tuned tocollectively display a uniform color.

It can be appreciated that the photonic microsphere system as discloseddoes not involve the nanoparticle assembly step, and therefore hasseveral advantages. These advantages include long-term stability ofoptical response, improved tolerance to environmental variances such asionic strength and solvent hydrophobicity, and greater convenience forincorporation into many liquid or solid matrices without the need ofcomplicated surface modification. For example, in accordance with anexemplary embodiment, it can be appreciated that the magnetochromaticmicrospheres can be incorporated into a matrix, which can reversiblychange between liquid and solid phases, to produce a switchable colordisplay system whose color information can be switched “on” and “off”multiple times by means of an applied magnetic field.

The composition for generating a structural color may be emulsified in asolvent when the composition for generating a structural color isdispersed in an immiscible solvent and mechanically stirred. Here, theimmiscible solvent may include a non-polar solvent and other solvents,which are not mixed with the composition for generating a structuralcolor. Specifically, the immiscible solvent may be a solvent notreactive under photopolymerization condition such as silicone oil,mineral oil or paraffin oil.

The technique capable of modulating a kind of the structural color usingthe external magnetic field may be suitable for applications such ascolor display, data storage devices, anti-counterfeiting materials andsensors.

In accordance with an exemplary embodiment, a display device including amicrosphere rotated with an external magnetic field is provided. Here,the microsphere may include an aligned structure of magneticnanoparticles. In addition, with the rotation of the microsphere, adiffraction angle of light passing through the microsphere may bechanged. In the display device, the microspheres may be dispersed in aphase-changeable matrix. When the phase-changeable matrix is in a liquidphase, the microspheres may be instantaneously rotated by adjusting adirection of the external magnetic field. And when the phase-changeablematrix is in a solid phase, the orientation of the microspheres may befixed. So it can be appreciated that the display device having on/offbistable states can be fabricated by embedding the microspheres in amatrix that can thermally switch between solid and liquid phases.

Hereinafter, exemplary embodiments described in the specification willbe described in detail with reference to drawings. FIG. 1 is a diagramof a composition for generating a structural. color in accordance withan exemplary embodiment. Referring to FIG. 1, a composition forgenerating a structural color 100 may include a curable material 110 andmagnetic nanoparticles 120 dispersed in the curable material 110.

The magnetic nanoparticles 120 may include a cluster 122 of magneticnanocrystals. The size of the magnetic nanoparticles 120 may be severaltens to hundreds of nanometers, and the size of the magneticnanocrystals may be several to several tens of nanometers. Examples ofthe magnetic nanocrystals may include a magnetic materials or a magneticalloys. The magnetic material or magnetic alloy may include at least oneselected from the group consisting of Co, Fe₂O₃, Fe₃O₄, CoFe₂O₄, MnO,MnFe₂O₄, CoCu, CoPt, FePt, CoSm, NiFe and NiFeCo.

The magnetic nanoparticles 120 may be superparamagnetic nanoparticlesincluding a superparamagnetic material. The superparamagnetic materialhas magnetism only in the presence of an external magnetic field, unlikea ferromagnetic material in which magnetism can be maintained without amagnetic field. Usually, when the particle size of a ferromagneticmaterial is several to several hundreds of nanometers, the ferromagneticmaterial may be phase-changed into a superparamagnetic material. Forexample, when iron oxide is in the size of approximately 10 nm, it mayhave superparamagnetism.

In addition, the magnetic nanoparticles 120 may be, as shown in FIG. 1,coated with a shell layer 124 surrounding a core formed in the cluster122 of magnetic nanocrystals. The shell layer 124 allows the magneticnanoparticles 120 to be evenly distributed in the curable material 110.Furthermore, to be described later. the shell layer 124 may stimulatesolvation repulsion on a surface of each magnetic nanoparticle 120 tooffset potent magnetic attraction between the magnetic nanoparticles120. For example, the shell layer 124 may include silica. When the shelllayer 124 is surface-modified with silica, a known sol-gel process maybe used.

In addition, the composition 100 for generating a structural color mayfurther include a hydrogen bonding solvent. As the hydrogen bondingsolvent, various alkanol solvents such as ethanol, isopropyl alcohol andethylene glycol may be used. Also, a solvation layer 126 surrounding themagnetic nanoparticle 120 may be formed. For example, as the solvationlayer 126 is formed due to an influence of a silanol (Si—OH) functionalgroup on a surface of the shell layer 124 having silica, a repulsionforce between the magnetic nanoparticles 120 may be induced. Accordingto one exemplary embodiment, the shell layer 124 and/or the solvationlayer 126 may not be present on the magnetic nanoparticles 120. In thiscase, an electrostatic force on the surface of the magneticnanoparticles 120 may act as a repulsion force.

As the magnetic nanoparticles 120 are mixed with the curable material110 and subjected to mechanical stirring or ultrasonic treatment, thecomposition 100 for generating a structural color may be prepared. Themagnetic nanoparticles 120 may be included in the curable material 110at a volume fraction of, for example, 0.01% to 20%. When the volumefraction of the magnetic nanoparticles 120 is less than 0.01%,reflectivity may be decreased, and when the volume fraction of themagnetic nanoparticles 120 is more than 20%, reflectivity may not beincreased any more.

The curable material 110 may serve as a dispersion medium stablydispersing the magnetic nanoparticles 120 forming a photonic crystal. Inaddition, as the inter-particle distance between the magneticnanoparticles 120 is fixed by crosslinking of the curable material 110,a certain structural color may be continuously maintained after amagnetic field is eliminated.

The curable material 110 may include a liquid-phase material such as amonomer, an oligomer or a polymer having a crosslinkable site for curingreaction. The curable material 110 may include a liquid-phasehydrophilic polymer capable of forming a hydrogel. A hydrophilic polymeris a polymer suitable for dispersing the magnetic nanoparticles 120 dueto its hydrophilic groups. When the hydrophilic polymer is crosslinkedby an appropriate energy source, thereby forming a hydrogel having athree-dimensional network structure, the magnetic nanoparticles 120 maybe fixed.

Examples of the curable material 110 capable of forming a hydrogel mayinclude a silicon-containing polymer, polyacrylamide, polyethyleneoxide, polyethylene glycol diacrylate, polypropylene glycol diacrylate,polyvinylpyrrolidone, polyvinyl alcohol, polyacrylate or a copolymerthereof. For example, since the curable material 110, polyethyleneglycol diacrylate (PEGDA), has an acrylate functional group at bothterminal ends of polyethylene glycol (PEG), the curable material 110 maybe crosslinked into a three-dimensional hydrogel via free radicalpolymerization. The curable material 110 may further include any type ofmedium which can be changed into a solid from a liquid.

The curable material 110 may further include an initiator, and theinitiator may induce free radical polymerization by an external energysource. The initiator may be an azo-based compound or a peroxide. Thecurable material 110 may further include a proper crosslinking agent,for example, N,N′-methylenebisacrylamide, methylenebismethacrylamide,ethylene glycol dimethacrylate, etc. The magnetic nanoparticles 120 mayaligned in the curable material 110 to generate structural colors underan external magnetic field.

FIG. 2 is a diagram for explaining a principle of generating structuralcolor. Referring to FIG. 2, when a magnetic field is not applied, themagnetic nanoparticles 120 are randomly dispersed in the curablematerial 110, but when a magnetic field is applied from a nearby magnet,the magnetic nanoparticles 120 may be aligned parallel to a direction ofthe magnetic field to form a photonic crystal, thereby emitting astructural color. The magnetic nanoparticles 120 aligned by the magneticfield may return to the non-aligned state when the magnetic field iseliminated. A photonic crystal is a material having a crystal structurecapable of controlling light. Photons (behaving as waves) propagatethrough this structure or not depending on their wavelength. Wavelengthsof light that are allowed to tray—are known as modes, and groups ofallowed modes form bands. Disallowed bands of wavelengths are calledphotonic band gaps. This gives rise to distinct optical phenomena suchas inhibition of spontaneous emission, high-reflecting omni-directionalmirrors and low-loss-waveguiding, amongst others. The magneticnanoparticles 120 present in a colloidal state may have an attractiveinteraction therebetween in the curable material 110 due to themagnetism when a magnetic field is applied outside, and also have arepulsive interaction caused by an electrostatic force and a solvationforce. By the balance between the attraction and the repulsion, themagnetic nanoparticles 120 are aligned at regular intervals, therebyforming a chain structure. Therefore, inter-particle distance d betweenthe aligned magnetic nanoparticles 120 may be determined by the magneticfield strength. As the magnetic field is stronger, the inter-particledistance d between the magnetic nanoparticles 120 aligned along thedirection of the magnetic field may be reduced. The inter-particledistance d may be several to several hundreds of nanometers with themagnetic field strength. With a lattice spacing in the photonic crystalis changed, the wavelength of reflected light limy be changed accordingto Bragg's law. As the magnetic field strength is increased, astructural color of a shorter wavelength region may be generated. As aresult, a wavelength of the reflected light may be determined by thestrength of a specific magnetic field. Unlike the conventional photoniccrystal reflected only at a certain wavelength, the photonic crystal mayexhibit an optical response that is fast, extensive and reversible withrespect to an external magnetic field. As the lattice spacing is changedwith the variation in the nearby magnetic field, the reflective lightwith a specific wavelength may be induced from external incident light.

The structural color may be dependent on a size of the magneticnanoparticle 120 as well as the magnetic field strength. For example, asFe₃O₄ magnetic nanoparticle 120 with a silica shell is increased in sizefrom approximately 120 nm to approximately 200 nm, the structural colormay shift from blue to red. However, it can be appreciated that thecolor or the diffraction wavelength is determined by not only themagnetic nanoparticle size, the silica shell layer, and magnetic fieldstrength, but also many other parameters such as the chemical nature ofthe curable material, the surface charge of the particle surface, andthe additives.

FIG. 3 is a diagram illustrating a procedure of fixing a photoniccrystal structure by curing a composition for generating a structuralcolor. As shown in FIG. 3, when the composition 100 for generating astructural color including the curable material 110 and magneticnanoparticles 120 is exposed to a magnetic field and UV rays areirradiated, a curing procedure progresses, thereby forming a solidmedium 110′. As a result, a photonic crystal structure of the magneticnanoparticles 120 may be fixed in the solid medium 110′. Thus, a coloredproduct having a structural color may be formed by using the composition100 for generating a structural color. The composition 100 forgenerating a structural color may be easily manufactured at a low cost,and exhibit reflected light with various wavelengths in the entireregion of visible light.

Physical/chemical properties of the solid medium 110′ may be modulatedby changing molecular weight of the curable material 110, aconcentration of an initiator, an irradiation time of UV rays, etc.

By the curing of the curable material 110, the solid medium 110′ may bein the form of a crosslinked polymer, A spacing between chains of thecrosslinked polymer having a network structure may be approximately 1 toseveral nanometers. Thus, provided that the conventional magneticnanoparticles 120 can have a size of approximately 150 to 170 nm, themagnetic nanoparticles 120 may be easily fixed. As a solvation layer 126is coated on a surface of the magnetic nanoparticles 120, the magneticnanoparticles 120 are spaced apart in a regular distance.

By using the composition 100 for generating a structural color describedabove, a microsphere having a structural color may be formed. Themagnetic nanoparticles 120 included in the microsphere having astructural color are arranged apart in a regular distance in a directionof at least one axis, thereby forming a chain structure. A wavelength oflight diffracted from external incident light is determined by a size ofthe regular distance, and therefore the microspheres may exhibit astructural color.

FIG. 4 is a schematic of a synthetic procedure for the magnetochromaticmicrospheres. In FIG. 4, a composition (Fe₃O₄@SiO₂/PEGDA) for generatinga structural color may be used to synthesize the magnetochromaticmicrospheres. It can be appreciated that besides silica, titania andsome polymer such as polystyrene and polymethylmethacrylate may be usedas a shell layer. The thickness of the silica coating may be controlledby controlling the amount of silane precursors or the catalyst.

The silica coated Fe₃O₄ superparamagnetic particles ay be dispersed in aliquid UV curable resin preferably containing mainly polyethyleneglycoldiacrylate (PEGDA) oligomers and a trace amount of photoinitiator2,2-Dimethoxy-2-phenylacetophenone (DMPA). It can be appreciated thatother suitable photocurable resins may be used including but not limitedto ethoxylated trimethylolpropane triacrylate (ETPTA),polyethyleneglycol diacrylate (PEGDA), 2-hydroxyethyl methacrylate(HEMA), methylmethacrylate (MMA), acrylamide (AAm) allylamine (AM)and/or any combination thereof. Alternatively, any medium capable ofbeing converted from liquid to solid such that ordered structures ofphotonic crystals are fixed within may be used.

The Fe₃O₄/PEGDA mixtures are then dispersed in a viscous non-polarsolvent (or immiscible liquid) such as silicone oil or mineral oil undermechanical stiffing, which leads to the formation of an emulsion. It canbe appreciated that besides silicone oil or mineral oil, the immiscibleliquid may be paraffin oil or any oil immiscible liquid with the curablesolution, and with appropriate density and inertness to polymerize.

Upon the application of an external magnetic field, thesuperparamagnetic particles self-assemble into ordered d structuresinside the emulsion droplets when the magnetically induced attractionreaches a balance with repulsive interactions including electrostaticand solvation forces.

In accordance with an exemplary embodiment, an immediate 365 nm UVillumination quickly polymerizes the PEGDA oligomers to transform theemulsion droplets into solid polymer microspheres, and at the same timepermanently fixes the periodic superparamagnetic structures. It can beappreciated that any suitable photolithography setup with UV lightpreferably in the range of approximately 240 nm (DUV) to 365 nm (I-Line)may be used with this system to fix the photonic structures in theresin. In addition, instead of using traditional mask-defined beampatterning which usually requires mechanical movement of the physicalmask, using the maskless lithography may enable high resolutionpatterning without any alignment error.

In accordance with an exemplary embodiment, microspheres with differentcolors may be obtained by controlling the periodicity of thesuperparamagnetic assembly through the variation of the externalmagnetic field during UV curing process. It can be appreciated that dueto the short-range nature of the solvation force, the range of colorthat can be produced from a single Fe₃O₄/PEGDA mixture may be limited.However, in accordance with exemplary embodiment, in order to producemicrospheres with largely different colors such as red and blue, Fe₃O₄particles with different initial sizes or with SiO₂ coatings ofdifferent thicknesses may be used. In accordance with an exemplaryembodiment, the diameter of the microspheres typically is preferably inthe range of approximately 1 μm to 300 μm, and more preferablyapproximately 10 μm to 100 μm, depending on the type of oil. and thespeed of mechanical stirring.

In accordance with an exemplary embodiment, the microspheres arepreferably larger than 10 μm, which will present a consistent color,which is mainly contributed by the straight photonic chain structuresinside the microsphere. However, it can be appreciated that microspheressmaller than 10 μm may be used. Once made uniformly in size, it can beappreciated that each of the microspheres should display the same colorwith magnetic tunability.

When the magnetic field is applied, the superparamagnetic nanoparticlesin the composition for generating a structural color may form a chainstructure parallel to the direction of the external magnetic field. FIG.5 is a scanning electron microscopy (SEM) image of Fe₃O₄ particlescoated with SiO₂ embedded in a PEGDA medium. FIG. 5 shows across-section of a spherical microsphere particle cut with a microtome.In FIG. 5, parallel particle chains with regular inter-particle spacingcan be easily observed. It can be appreciated that the separationbetween neighboring chains is typically on the order of a fewmicrometers due to the strong inter-chain repulsion induced by theexternal field.

FIG. 6 shows schematic illustrations and optical microscopy images forthe magnetochromatic effect caused by rotating the chain-like photonicstructures in magnetic fields. Referring to FIG. 6, the diffraction ofthe microspheres dispersed in a liquid can be conveniently switchedbetween “on” and “off” states by using the external magnetic field. In avertical field, the particle chains stand straight so that theirdiffraction is turned “on” and the corresponding color can be observedfrom the top. Each bright green dot in the optical microscopy imageactually represents one vertically aligned particle chain. On thecontrary, when the field is switched horizontally, the microspheres areforced to rotate 90° to lay down the particle chains so that thediffraction is turned off and microspheres show the native brown colorof iron oxide. It can be appreciated that the particle chains can bedirectly observed by careful inspection of the microspheres throughmicroscopy. The rotation of the microspheres is instant, andsynchronized with the natural movement of external fields.

FIGS. 7A-13F demonstrate the complete on/off switching ofmagnetochromatic microspheres that originally diffract blue, green andred light. These microspheres are synthesized by starting withsuperparamagnetic particles with average diameters of 127, 154, 197 nm.It can be appreciated that by mixing of RGB (Red, Green and Blue)microspheres in various ratios can produce a great number of colors thatcan be collectively perceived by human eyes.

According to a size of the superparamagnetic nanoparticles in themicrosphere particle, a wavelength showing the maximum reflectivity isdifferent. In the case of (a), the microsphere particle is blue, in thecase of (c), the microsphere particle is green, and in the case of (e),the microsphere particle is red.

FIG. 8 is a schematic illustration of the experimental setup forstudying the angular dependence of the diffraction property of themagnetochromatic microspheres. Depending on the direction of theexternal magnetic field, the particle chains can be suspended at anyintermediate stage between the on/off states with a specific tiltingangle (θ). While the magnetic field is tuned within the planeconstructed by the incident light and back scattered light, the from anisolated microsphere is recorded correspondingly by the spectrometer, asschematically shown in FIG. 8.

FIG. 9 is a reflection spectrum and corresponding digital photo recordedfrom a single Fe₃O₄@SiO₂/PEGDA microsphere at different tilting angles.In FIG. 9, the dependence of diffraction peak wavelength (λ) andintensity (R) on the tilting angle (θ) using an optical microscopecoupled with a spectrometer is shown. It can be appreciated that thediffraction peak blue-shifts with decreasing intensity when the magneticfield direction is manipulated away from the angular bisector ofincident light and back scattered light (θ≈14.5°). FIG. 9 shows thespectra and corresponding microscopy images when the angle θ is tiltedfrom +10° to −30°. Such a change in the diffraction peak position andintensity closely resembles the characteristics of a one-dimensionalBragg photonic crystal, as proven by the close match between theexperimental results and theoretical simulations. Beyond −30°, thediffraction intensity is very low so that the photonic state of themicrosphere can be practically considered as “off”.

In accordance with an exemplary embodiment, the average size of themicrospheres can be controlled using the simple dispersing processthrough the choices of the oil type and the speed of mechanicalstirring. It can be appreciated that several methods including thoseusing microfluidic devices are available to produce monodispersedmicrodroplets. In general, using high speed stirring and viscous oilsleads to the formation of smaller emulsion droplets. The microspheresprepared in mineral oils have average diameters above 50 μm, and thoseprepared in silicone oils have average diameters less than 30 μm.

FIG. 10A illustrates dark-field optical microscopy images of a series ofFe₃O₄@SiO₂/PEGDA microspheres with diameters from approximately 150 to 4μm, and wherein the larger microspheres were fabricated in mineral oiland smaller ones in silicone oil. Vertical external fields are appliedso that these microspheres are all at the “on” state. Microsphereslarger than 10 μm containing particle chains with spacing such that theyreflect red light all display the expected red dots, which contribute tothe overall production of red color, can be clearly observed inside themicrospheres 10 μm and smaller containing similarly spaced particlechains, fewer red dots can be observed in the center. Instead,contribution of the diffraction from the edge to the overall color ofthe microspheres gradually increases, with a progressive blue-shift fromorange to yellow and eventually yellow-green as the microsphere size isreduced. This phenomenon can be explained by the unique self-assemblybehavior of superparamagnetic particles in the PEGDA droplets.

FIGS. 10B-10D show the top-view and side-view SEM images of the typicalmicrosphers, suggesting that the superparamagnetic particle chains arenot only embedded inside the microspheres in the form of straightstrings but also laid on the curved surface along the longitudinaldirection. The “bent” assembly of superparamagnetic particles on themicrosphere surface can be attributed to the combined effect of thespherical confinement of the emulsion droplets and the magneticallyinduced strong repulsive force perpendicular to the direction of theexternal field. The bent surface assemblies can be viewed as chainstilted from the vertical direction with the degree of tilting determinedby the curvature of the microspheres. As the microspheres becomesmaller, the curvature becomes larger and tilting angle increases,leading to a blue-shift of the diffraction. Additionally, the highersurface to volume ratio of smaller microspheres may also increase theratio of surface chains to embedded ones and eventually change theoverall diffracted color of the spheres. For microspheres larger than 10μm, the embedded straight assemblies dominate and the bending of thesurface assemblies is small, so that the microspheres show uniformcolors.

The optical response of the microspheres to the external magnetic fieldwas characterized by the switching threshold of field strength andswitching frequency, which describe how strong of an external magneticfield is required to rate the microspheres and how fast the microspheresrespond to the changes in the magnetic field, respectively. First, a lowconcentration of microspheres dispersed in a density matchedsolvent—PEGDA liquid were used to measure the switching threshold. Thedispersion was sandwiched between two hydrophobic glass slides to avoidadhesion to the glass substrate. With increasing magnetic fieldstrength, the microspheres were gradually turned “on” and digital photoswere taken after approximately 5 seconds of every change in the fieldstrength.

FIGS. 11A and 11B show the statistic diagrams of the percentage ofmicrospheres (counted in viewable area) that have been turned “on” in anincreasing field for two samples with different loading of the magneticmaterials. The corresponding accumulative curves are plotted from thediagrams. It has been found that the loading of the magnetic materialsin the microspheres, and not the sphere size is one of the factors,which determines the switching threshold of the field strength. Formicrospheres with low magnetic loading (8 mg Fe₃O₄/mL PEGDA), 80% ofthem can be turned on in a magnetic field of approximately 180 Gauss,while for microspheres containing more superparamagnetic particles (16mg Fe3O4/mL PEGDA), only a 100 Gauss magnetic field is required to turnon the numbers of spheres.

The switching of diffraction could be accomplished rapidly (i.e., lessthan approximately 1 second (<1 s)) in a sufficiently strong magneticfield. Turning frequency of the microspheres was measured with a testplatform built with a halogen light source, a spectrometer and arotating magnet unit with geared DC motor. The rotating plate with NSand SN magnets standing alternately will produce a periodical vertical(1100-1200 Gauss) and horizontal magnetic field (300-400 Gauss), whosefrequency can be simply controlled by the rotating speed of the plate.

FIGS. 12A-D illustrate schematic diagrams of the optical response ofFe₃O₄@SiO₂/PEGDA microspheres in a (FIGS. 12A, 12B) 1.22 and (FIGS. 12C,12D) 313 Hz vertical/horizontal alternating magnetic field, whereinH_(s)/H₀ is the ratio of reflection with H field to that without Hfield. FIGS. 12A-12D demonstrate that the photonic microspheres can berotated quickly. It can be noted that the rotating amplitude graduallydecreases with the increase of turning frequency, primarily due to therelatively weak horizontal field strength. In addition, it can beappreciated that when the frequency is higher than approximately 7 Hz,the rotation of microspheres cannot catch up with the external fieldvariation so that they seem to simply vibrate around the vertical stateand the diffraction remains on all the time. In accordance with anexemplary embodiment, the switching frequency can be further improvedwhet the microspheres are dispersed in a less viscous solvent or tunedin magnetic fields with higher strengths.

In accordance h an exemplary embodiment, the incorporation of photoniccrystals into microspheres allows tuning of the photonic property bysimply controlling the sphere orientation, making it very convenient tocreate bistable states that are required for a plurality of applicationssuch as displays. For example, a simple switchable color display systemin which the color information can be rewritten multiple times by meansof the magnetic field. The basic idea is to create bistable states byembedding the microspheres into a matrix that can be switched betweenliquid and solid states.

In accordance with an exemplary embodiment, long chain hydrocarbons andshort chain polymers, such as paraffin and poly(ethylene glycol), havemelting points slightly above room temperature. When heated, the matrixmaterial melts, allowing the display of colors by aligning themicrospheres using magnetic fields. When the system is cooled to roomtemperature, the matrix solidifies and the orientation of microspheresis frozen so that the color information remains for long time withoutthe need of additional energy. It can be appreciated that an externalmagnetic field cannot alter their color once the orientation ofmicrospheres is fixed by the matrix. Reheating the matrix materials,however, will erase the particular color by randomizing the orientationof the microspheres or by magnetically reorienting the microspheres to acompletely “off” state.

FIGS. 13A-13I show three examples of such displays fabricated byembedding the microspheres in polyethylene glycol (PEG, Mw=1500) films,which can be melted at approximately 46° C. In FIGS. 13A-13I, thediffraction is switched on (FIGS. 13A, 13D, 13G) or off (FIGS. 13B, 13E,13H) by melting the PEG matrix, rotating the microspheres with amagnetic field, and finally cooling down the PEG matrix to lock thesphere orientation such that bistable states can therefore be maintainedin the absence of magnetic fields, and the corresponding reflectionspectra FIGS. 13C, 13F, 13I) display diffraction peaks at the “on” stageand none at the “off” stage. In FIGS. 13A-13C, the superparamagneticnanoparticles are in a size of 197 nm, and the microsphere particle isred. In FIGS. 13D, 13E, and 13F, the superparamagnetic nanoparticles arein a size of 154 nm, and the microsphere particle is green. FIGS. 13G,13H, and 13I, the superparamagnetic nanoparticles are in a size of 127nm, and the microsphere particle is blue.

The comparison of digital photos and reflection spectra clearlydemonstrates two stable diffractive states at room temperature,suggesting the possible applications of such systems as economical andrewritable color display units.

It can be appreciated that the magnetochromatic microspheres can beprepared through a simultaneous magnetic assembly and UV curing processin an emulsion system. In accordance with an exemplary embodiment,superparamagnetic Fe₃O₄@SiO₂ colloidal particles are self-organized intoordered structures inside emulsion droplets of UV curable resin,followed by an immediate UV curing process to polymerize the dropletsand fix the ordered structures. In addition, it can be appreciated thatby rotating the microspheres, the orientation of the magnetic chains canbe controlled, and thereby exhibit the diffractive colors. In addition,a plurality of copies of the microspheres can be produced using theprocess, and can be tuned by external fields to collectively displayuniform colors. The excellent stability, good compatibility withdispersion media, and the capability of fast on/off switching if thediffraction by magnetic fields, also make the system suitable forapplications such as color displays, signage, bio- and chemicaldetection, and magnetic field sensing.

In accordance with an exemplary embodiment, as the size of the magneticparticle increases, the color red shifts (or the diffraction wavelengthincreases). As the thickness of silica coating increases, the color redshifts (or the diffraction wavelength increases). As the magnetic fieldstrength increases, the color blue shifts (or the diffraction wavelengthdecreases). However, it can be appreciated that the color or thediffraction wavelength is determined by not only the magnetic particlesize, the silica coating (or coating medium), and magnetic fieldstrength, but also many other parameters such as the chemical nature ofthe resin, the surface charge of the particle surface, and theadditives.

In accordance with an exemplary embodiment, the relation of the colors(Red, Green & Blue) to the three parameters (size of magnetic particle,thickness of silica coating, magnetic field strength) is as follows, asthe overall size of Fe₃O₄@SiO₂ colloids increase from about 120 nm to200 nm, the color shifts from blue to red. As the magnetic fieldstrength increase, the color would blue shift.

In accordance with an exemplary embodiment, the magnetic fieldpreferably is in the range of approximately 100 Gauss to approximately400 Gauss. It can also be appreciated that the magnetic field strengthrequired to rotate the microspheres is dependent on an amount of themagnetic particles in each of the microspheres. As the amount ofmagnetic content within a composite, which is defined as magneticdensity, the more magnetic content, less magnetic field is required torotate the microspheres.

In accordance with an exemplary embodiment, the method and systems asdisclosed herein, microspheres can be incorporated into a display devicewherein very small quanta of microspheres can be locally manipulated tochange color or to create on-off color using an integrated micromagneticactuator to produce local magnetic flux in the area from several to tensof micrometers.

In accordance with an exemplary embodiment, it can be appreciated thatthe ordered structures in the micromagnetospheres are composed ofparallel 1D chains of magnetic particles, their spacing determined bythe balance of the attractive and repulsive forces, which in turn areaffected by the external magnetic field. In addition, it can beappreciated that the colors exhibited by the magnetic particles insolution, or fixed, are created by the ordered structures describedabove.

The foregoing is illustrative of the present disclosure and is not to beconstrued as limiting thereof. Although numerous embodiments of thepresent disclosure have been described, those skilled in the art willreadily appreciate that many modifications are possible in theembodiments without materially departing from the novel teachings andadvantages of the present disclosure. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure as defined in the claims. Therefore, it is to beunderstood that the foregoing is illustrative of the present disclosureand is not to be construed as limited to the specific embodimentsdisclosed, and that modifications to the disclosed embodiments, as wellas other embodiments, are intended to be included within the scope ofthe appended claims. The present disclosure is defined by the followingclaims, with equivalents of the claims to be included therein.

What is claimed is:
 1. A microsphere having a structural colorcomprising: a solid matrix; and magnetic nanoparticles aligned toexhibit a structural color within the solid matrix, wherein the solidmatrix and the magnetic nanoparticles form a microsphere, wherein themicrosphere is made by the steps of: providing a composition, whereinthe composition includes a curable material and the magneticnanoparticles dispersed in the curable material; adding the compositionto an immiscible solvent to form an emulsion including sphericaldroplets of the curable material dispersed in the immiscible solvent,each spherical droplet including the magnetic nanoparticles dispersedtherein; applying a magnetic field to the emulsion to align the magneticnanoparticles located in each spherical droplet of the curable material;and curing the spherical droplets dispersed in the immiscible solventwhile applying the magnetic field to the emulsion to i) fix the alignedmagnetic nanoparticles in each spherical droplet and ii) form themicrosphere which is cured and dispersed in the immiscible solvent,wherein the microsphere includes the aligned magnetic nanoparticles inthe cured material and has the structural color generated by adiffraction of light passing through the microsphere, wherein themicrosphere contains an ordered structure of a photonic crystal due tothe alignment of the magnetic nanoparticles, and a diffraction angle ofthe light passing through the microsphere is changed with rotation ofthe microsphere.
 2. The microsphere according to claim 1, wherein themagnetic nanoparticles include a superparamagnetic material.
 3. Themicrosphere according to claim 1, wherein the magnetic nanoparticleshave a structure coated with a shell layer surrounding a core formed ina cluster of magnetic nanocrystals.
 4. The microsphere according toclaim 3, further comprising a solvation layer coating a surface of themagnetic nanoparticles.
 5. The microsphere according to claim 1, whereinthe microsphere is dispersed in a phase-changeable matrix.
 6. A displaydevice comprising a microsphere rotated by an external magnetic field,wherein the microsphere having a structural color comprising: a solidmatrix; and magnetic nanoparticles aligned to exhibit a structural colorwithin the solid matrix, wherein the solid matrix and the magneticnanoparticles form a microsphere, wherein the microsphere is made by thesteps of: providing a composition, wherein the composition includes acurable material and the magnetic nanoparticles dispersed in the curablematerial: adding the composition to an immiscible solvent to form anemulsion including spherical droplets of the curable material dispersedin the immiscible solvent each spherical droplet including the magneticnanoparticles dispersed therein: applying a magnetic field to theemulsion to align the magnetic nanoparticles located in each sphericaldroplet of the curable material; and curing the spherical dropletsdispersed in the immiscible solvent while applying the magnetic field tothe emulsion to i) fix the aligned magnetic nanoparticles in eachspherical droplet and ii) form the microsphere which is cured anddispersed in the immiscible solvent wherein the microsphere includes thealigned magnetic nanoparticles in the cured material and has thestructural color generated by a diffraction of light passing through themicrosphere, wherein the microsphere contains an ordered structure of aphotonic crystal due to alignment of magnetic nanoparticles, and adiffraction angle of light passing through the microsphere is changedwith rotation of the microsphere.
 7. The display device according toclaim 6, wherein the magnetic nanoparticles include a superparamagneticmaterial.
 8. The display device according to claim 6, wherein themagnetic nanoparticles have a structure coated with a shell layersurrounding a core formed in a cluster of magnetic nanocrystals.
 9. Themicrosphere according to claim 6, further comprising a solvation layercoating a surface of the magnetic nanoparticles.
 10. The display deviceaccording to claim 6, wherein the microsphere is dispersed in aphase-changeable matrix.
 11. The display device according to claim 6,wherein, when a phase-changeable matrix is in a liquid phase, adjustingan angle of the external magnetic field changes an orientation of themicrosphere.